1. Field of the Invention
This invention relates to a method and a device for the control and setting of the redox state of redox fining agents in a glass melt.
2. Background Information
With reference to glass melts, fining or refining is the name given to the process of removing gas bubbles from the glass melt. To achieve the maximum freedom from foreign gas and bubbles, the molten batch must be thoroughly mixed and degasified.
Glass melting furnaces can be regarded as chemical reactors. Raw materials are converted by a large number of solid state or solid/liquid state reactions, in these reactors into a homogeneous melt, without unmolten material, without bubbles or other inhomogeneities. Generally, fossil fuel flames directly heat the melt, mainly by radiatiative heat transfer. Respective flows are determined by free convection due to temperature gradients and forced convection due to stirring and the pull. The raw materials are heated up to 500-600 degrees Celsius before the first solid state reactions take place. Secondly primary melt phases appear at about 1000 degrees Celsius. Sand grains dissolve or react in these liquid phases forming a viscous melt. Eventually all the raw materials are completely molten at a temperature of about 1300 degrees Celsius.
The melt contains dissolved gases and large numbers of small bubbles. By adding a fining agent to the batch of raw material, the bubbles and gases can be removed. Due to chemical reactions, often redox reactions, dissolved fining agents start to decompose at relatively high temperatures. The decomposition of the fining agents produces gases which diffuse in the existing bubbles. These bubbles will grow and will remove the dissolved gases present in the melt. Large bubbles will ascend in the melt up to the surface sometimes forming foam layers.
After the removal of the bubbles, homogenization and thermal conditioning takes place, this is mainly a physical process but diffusion will contribute to the elimination of concentration profiles.
All these processes: melting-in, dissolution of sand grains, degassing, homogenization take place in one space.
Residence time analysis shows that the glass melt can follow very different flow trajectories. The minimum residence time is about 15 up to 25% of the mean residence time but the maximum residence time can be several times the mean residence time. The glass melt volume will be exposed to a certain temperature history during melting, fining and homogenization dependent on the trajectory.
Due to the broad residence time distribution and fluctuations in parameters which influence the melting process, the glass quality is often far from stable and the part of the glass melt following the critical trajectory in the furnace (low residence time value and low temperature levels) may spoil the glass melt delivered to the forming sections of the glass production line.
In a float process, the raw materials for the glass are typically added together in a furnace, where they are mixed and melted. As the various glass constituents melt, they release gas. That is potentially a problem because some of the gas may remain trapped as small bubbles in the glass. If the bubbles remain in the final product, they result in visible imperfections in the glass. To avoid that problem, various melting and fining aids are added to the glass mixture in the furnace. Sodium sulfate (Na2SO4) is one such additive. In the mixture, much of the sulfate decomposes into sulfur dioxide and oxygen. Those gases cause the trapped gas bubbles to dissipate, leaving few visible imperfections in the glass. Some of the sulfate introduced into the batch remains dissolved in the glass composition, but it is colorless and has no effect on the transmittance properties of the glass.
The behavior of gases and or bubbles in a glass melt and the removal of gases and bubbles is described, for example, in “Glastechnische Fabrikationsfehler” [Manufacturing Defects in Industrial Glass], edited by H. Jebsen-Marwedel and R. Brückner, 3rd Edition, 1980, Springer Verlag, on pages 195 ff.
The fining methods used are most frequently chemical. The principle of these methods is that compounds are added to the glass melt that decompose and give off gases or compounds that are volatile at elevated temperatures or compounds that give off gases in an equilibrium reaction at elevated temperatures.
The latter group of substances comprises redox fining agents such as, for example, arsenic oxide and antimony oxide, but also SnO2, CeO2, Fe2O3, ZnO, TiO2, V2O5, MoO3, WO3, Bi2O5, Cr2O3 and MnO. The redox fining agents used are polyvalent ions that exist in at least two oxidation stages that are in a temperature-dependent equilibrium with each other, whereby a gas, usually oxygen, is released at high temperatures.
The redox equilibrium of the substance dissolved in the glass melt can be illustrated with the example of arsenic oxide by Equation (I):As2O5⇄As2O3+O2↑
The equilibrium constant K in Equation (I) can be formulated as in Equation (II):       K    ⁢                   ⁢          (      T      )        =                              a          ⁢          As                2            ⁢                        O          3                ·                                   ⁢        p            ⁢                           ⁢              O        2                    a      ⁢                           ⁢              As        2            ⁢              O        5            
In this equation, aAs2O3 and aAs2O5 are the activities of the arsenic trixoide or arsenic pentoxide and pO2 is the fugacity of the oxygen.
The equilibrium constant K is dependent to a very great extent on the temperature, and a defined oxygen fugacity pO2 can be set by mans of the temperature and the activity of the oxide arsenic compounds.
There are essentially three types of chemical fining:                1) a primary fining effect, during which the gases that form during the decomposition of the added fining agents, e.g. oxygen gas from redox fining agents, diffuse into the bubbles that originate during the decomposition of the batch, such as, for example CO2, N2, H2O, NO and/or NO bubbles;        2) a secondary fining effect in which there is a degasification of the glass melt, during which there is a spontaneous formation of gas bubbles from the added fining agents, e.g. O2 bubbles from redox fining agents. Foreign gases such as CO2, H2O, N2, NO and/or NO2 can diffuse into these fining bubbles, even if their partial pressure is less than 105 Pa, and        3) a resorption effect, during which any expanded bubbles that originated as described in 1) or 2) above and are still in the melt when the temperature decreases are dissolved, for example by the oxygen, for example at redox equilibrium (I) by shifting the equilibrium to the side of the educt.        
The release of the fining gases frequently occurs as early as during the melting and the fining gases are no longer available for the secondary fining effect. Only the primary fining effect takes place. Conventional redox fining agents such as As2O5 or Sb2O5 have an effective fining oxygen release between 1150 degrees Celsius and 1500 degrees Celsius with a maximum at 1220 degrees Celsius to 1250 degrees Celsius, whereby the respective oxygen release, in addition to the temperature, is essentially a function of the composition of the glass and the composition of the fining agent (one or more fining agents). In particular for glass with a high melting point, larger amounts of fining agent than would normally be necessary must be used to achieve any fining effect at all. The large amounts of fining agent are disadvantageous, particularly with arsenic and-antimony oxide, because they are highly toxic and very expensive. Moreover, the addition of fining agents can have an undesirable effect on the characteristics of the glass and increase the manufacturing costs—because the compounds in question are generally expensive.
For an optimal fining using redox fining agents, therefore, an attempt is made to keep the greatest possible percentage of the fining agent in the higher oxidation stage beyond the melting process. With some fining agents, the higher oxidation stage is not attractive for commercial use, and with other fining agents the higher oxidation stage is to a large extent reduced as early as in the batch. For the oxidation of the one fining agent or to prevent the reduction of the other fining agent, nitrate compounds are added to the batch, which nitrate compounds decompose and release oxygen.
One disadvantage of this method is that nitrates are relatively expensive components of the batch and also release environmentally harmful NOx compounds when they decompose. When they are reduced to pure nitrogen, nitrates are a source of nitrogen bubbles, which are very difficult to remove. When pure oxyfuel heating is used, they are in fact the only source for nitrogen bubbles from the melting process. The difficulty of removal of nitrogen bubbles lies in their very low solubility and diffusion constant in conventional industrial glass melts.